1. Field of the Invention
The present invention relates to a rangefinder using ultrasonic waves.
2. Prior Arts
Various rangefinders have been proposed for ranging by a time difference between a sending time and a receiving time of ultrasonic waves. The conventional rangefinders have the following problems as follows. In general, a receiving microphone for the ultrasonic waves, which is disposed adjacent to an ultrasonic wave speaker, receives direct waves which are other than the ultrasonic reflection waves from a ranging object, and travelling through short space from the speaker to the microphone, although the strength of the direct waves coming back from various directions depends on directivity of the sending speaker. When the sending speaker and the receiving microphone are both mounted on a same supporter or a frame, solid waves are also transmitted through a direct path in the supporter from the speaker to the microphone. In the conventional rangefinders, an amplification degree of a signal for the reflection waves is selected to be sufficient for receiving a small intensity signal produced by the reflection waves from the object of a maximum range distance L.sub.max. But in such a case, the direct waves may come back to the rangefinder earlier than the correct reflection waves and even small direct waves may be amplified well to result in a pseudo-signal which is erroneously used for the time difference determination. This problem would be resolved by decreasing the amplification degree for the direct waves, but paradoxically the maximum range distance also becomes shorter by such a measure. Therefore, the conventional rangefinders have been subject to the problems of the direct waves. The conventional rangefinders have a further drawback that circuit configurations are complicated.